When an electromagnetic contactor is used to start up an induction motor from rest, during acceleration the motor draws a starting current from the supply that is between six and ten times motor full load current (FLC), depending on the size and construction of the motor. As the motor approaches full speed, the current falls to the full load value, or less if the load on the motor is less than full load.
However, a number of additional undesirable phenomena occur which are caused by the sudden connection of the supply to the motor. Most obviously, during the first fraction of a second or so following switch-on (increasing to many seconds for large motors), there is a severe oscillatory pulsation in torque generated by the motor. This can be heard and felt as a strong vibration which imposes high mechanical stress on the whole of a drive train to which the motor is connected, especially on shaft couplings, gearboxes, bearings, and on stator windings though the reaction force that they experience. The peaks in the pulsating torque can be both positive and negative, and many times the maximum torque experienced under normal running. This variation in torque can be a significant factor in causing breakdown, especially in motors subject to frequent starting.
Less obvious, but just as serious, is the fact that during the transient period of torque pulsation, supply current peaks can be up to twice the expected steady-state locked rotor starting current. This abnormally high current is known as surge current, and can cause problems for motor protection. Generally, motor starters combine a contactor with overload protection to disconnect the motor if it draws excessive current. The overload mechanism must allow for the high starting current without disconnecting the motor prematurely, but nevertheless be able to shut down the motor during running if it becomes overloaded and draws more than only 110% of full load current. With high-efficiency motors, surge current can reach eighteen times FLC, which complicates setting of overload relays and breakers to allow starting yet still provide adequate running protection.